1. Field of the Invention
This invention relates to fracturing zones in a well, and more particularly, to a method and apparatus which allows multiple zones to be treated with one trip of the apparatus into the hole and which does not require hydraulic fracturing to be removed from each zone separately.
2. Description of the Prior Art
Hydraulic fracturing is the parting of a desired section of a well formation or zone by an application of hydraulic pressure. A fracture made in this manner may be extended from the wellbore by continued pumping. Extended distance will depend on several factors such as injection rates, formation or zone characteristics, fracturing fluid properties and injected volume of fluid.
Selected particles, added to the fracturing fluid, are transported into the fracture. These act as propping agents to hold the fracture open when the applied pressure is dissipated, thus leaving a channel of high flow capacity connected to the wellbore.
Initially, fracturing was applied primarily to old wells, but now a majority of the treatments performed are on new ones. Many new fields and field extensions have resulted from the application of this production stimulation technique.
Research dealing with theory, methods and materials has resulted in a wide selection of fracturing fluids, additives and equipment to satisfy the many well conditions encountered.
Procedures have been developed to enhance the possibility of obtaining additional fractures during a single treatment of a well. These techniques are known as Multi-Frac. They are especially applicable on wells having multiple pay zones or long producing zones.
Any of a variety of known fracturing fluids may be employed to produce a fracture in the well. Then, that fracture is bridged or sealed. Additional fracturing fluid is then diverted to other parts of the zone, or to other zones, to produce another fracture. Repetition of this procedure has made varied numbers of fractures, as indicated by surveys conducted before and after the treatment. Granular type materials of specific size and characteristics have proven to be very effective for quick sealing and bridging fractures, thus diverting the fluid to other portions of the formation. These are called bridging agents.
The assignee of the present invention has developed a multiple-stage fracturing system which reduces elapsed times between stages, thus allowing operators to reduce rig time, completion time and expense. The system utilizes one or more tubing or casing baffles positioned in the completion string between the zones to be stimulated. Each zone may be perforated, fractured and then temporarily isolated with a bridging ball while a shallower zone is being completed in like manner. Thus, a two-, three-, four- or five-stage fracturing operation usually can be completed in a semi-continuous manner allowing for the short time delay required to perforate each zone before treatment.
Casing or tubing baffles installed between zones in the completion string have graduated internal openings. Sealing balls are selected and mated with the baffles so that the smaller diameter balls or plugs will pass through the larger ID baffles that are installed at the more shallow depths. Some of the balls are of such density that they usually are expelled from the tubing with the flow of fluids and gas when the well is opened to test or production.
Other, more conventional methods utilize setting a plug below a zone and positioning a fracturing tool thereabove which includes a packer. The tool is then removed from the wellbore and another bridge plug positioned below the next higher zone, and the tool utilized again to fracture the formation. This can be repeated as many times as necessary. This system has the disadvantage of having to remove the fracturing tool at each formation so that another plug can be set.
The apparatus of the present invention solves these problems by providing a tool which can be utilized to fracture multiple zones without requiring that the tool be removed from each zone separately. Rather, it can be used to fracture multiple zones in a single trip.